1. Field of the Invention
The present invention relates to a structure for the bearings of a throttle shaft in a carburetor, and particularly to a carburetor that mixes lubricating oil with an air-fuel mixture supplied to a combustion chamber in order to lubricate respective parts of the engine.
2. Description of Background Art
An engine is provided with a carburetor for charging air with a spray of liquid fuel, and an engine speed is changed by varying the amount of the opening of a throttle valve of the carburetor.
The carburetor includes the throttle valve that is positioned in a barrel, and can be freely opened or closed via a throttle shaft. In response to the rider's throttle operation, the throttle shaft is turned, so that the carburetor changes the opening amount of the throttle valve, and controls the amount of an air-fuel mixture to be supplied to the engine.
This carburetor is usually applied to various kinds of engines. For instance, it is used for supplying the air-fuel mixture to a compact and high output two-cycle engine in a motorcycle, a small leisure boat, or the like.
In the foregoing engine, various measures such as sealing members have been provided in order to prevent water from entering into the engine. Specifically, such measures are essential in a motorcycle whose engine is often exposed to rainwater or a boat whose engine is affected by bilge.
However, the carburetor includes a throttle shaft which is rotatably attached to its body and is connected to a throttle wire for transmitting a throttle operation force, and an accelerating pump operating in response to the rotation of the throttle shaft. The throttle shaft has to be exposed from the carburetor body. Therefore, even when sealing members are attached to bearings of the throttle shaft, external water may enter into the carburetor via clearances that are caused by the rotating throttle shaft and the bearings. This means that water enters into the engine together with an air-fuel mixture.
The foregoing situation will be described in detail with reference to a carburetor for a two-cycle boat engine.
In a small leisure boat or the like, water usually enters into an engine room since water is splashed while propelling the boat through the water. Therefore, water tends to enter into the bearings of the throttle shaft not only from the exterior but also from the interior (i.e., a barrel passage), which often causes the bearings to rust or to become clogged with the salt that is in seawater.
In order to overcome the foregoing problem, sealing members are usually attached to the bearings. When the sealing members are simply provided on an outer side of the bearings, it is impossible to prevent water, which is contained in the intake air, from entering into the bearings of the throttle shaft from the barrel passage (from the interior), because of capillary action. Therefore, the bearings have to be protected at the inner and outer sides by sealing members. Once such a sealing structure is utilized, water that is already present in the bearings, cannot be removed. This means that the bearings easily gather rust.
Under the foregoing situation, carburetors for small boats do not usually have the sealing members. Large clearances are provided in the bearings in order to allow water to leave from the bearings.
However, when the bearings have large clearances, the throttle shaft is very shaky when it turns, which means that the carburetor tends to control the amount of fuel to be supplied to the engine with reduced precision, and becomes less durable. Further, it is necessary to precisely control the amount of fuel to be supplied in order to promote exhaust purification, but it difficult for such a carburetor to meet this requirement.